1. Field of the Invention
The present invention relates to a semiconductor device such as a light emitting diode (LED) chip, an LED print head that uses the semiconductor device, and an image forming apparatus that incorporates the LED print head.
2. Description of the Related Art
FIG. 17 is a schematic perspective view illustrating a portion of a conventional LED print head 500. FIG. 18 is a top view illustrating a portion of an LED array chip 502 that can be applied to the LED print head in FIG. 17. Referring to FIG. 17, the print head 500 includes a substrate 501 on which LED array chips 502 and drive IC chips 504 are mounted. The LED array chip 502 and drive IC chip 504 are electrically connected through bonding wires 506. The LED array chip 502 includes light emitting elements 507 as shown in FIG. 18, aligned in a line. Each of the light emitting elements includes a electrode 508, which is electrically connected to an electrode pad 503.
FIG. 19 is a cross sectional view taken along a line H-H of FIG. 18. Referring to FIG. 19, a back surface electrode 510 is a common electrode. A GaAsP epi-layer 512 and a GaAs substrate 511 are common to individual light-emitting regions (Zn diffusion region 513). An interlayer dielectric film 514 is formed on the GaAsP epi-layer 512 and the individual light-emitting region 513. Current flows from the individual electrode 508 into the Zn diffusion region 513, then passing through a pn junction into the GaAsP epi-layer 512 and GaAs substrate 511, and finally reaching the common electrode (back surface electrode 510). When more than one LED is energized, current flows through the individual electrode 508 of each LED into a corresponding pn junction, and then through the GaAsP epi-layer 512 and GaAs substrate 511. Therefore, the lager the number of energized LEDs, the larger the current flowing through the common portions.
Thus, when the common portions have a high resistance (resistance of semiconductor and the contact resistance between the semiconductor layer and the electrode) or when the current flowing through an LED is much larger than those flowing through other LEDs, the potential in the vicinity of the pn junction of individual LEDs goes up due to more than one LED is energized and the potential difference between the individual electrodes and common electrode varies. The LEDs are driven by drive ICs. If the load on the drive IC changes by more than a certain value, the output current of the drive IC changes by a large amount. If the resistance of common portion (i.e., semiconductor resistance and the contact resistance between the semiconductor and electrode) is relatively high, the power of light emitted from the LEDs varies by a large amount depending on the number of energized LEDs.